The present invention relates to digital still or video cameras using a solid state image pickup device and more particularly to methods and systems used to detect and correct defective pixels obtained from a solid state image pickup device.
The foundation of digital imaging is the image sensor. Just as in a traditional camera, light enters a digital still or video camera through a lens controlled by a shutter. When the shutter opens, rather than exposing film, the digital camera collects light on an image sensor—a solid state electronic device. Image sensors contain a grid of microscopic photosites or pixels. In a camera, each photosite on the image sensor may represents one pixel of a digitized image. Two types of image sensors that are quite prevalent in the digital imaging field are the charge-coupled device (CCD) and the complimentary metal oxide semiconductor (CMOS) image sensor arrays. These image sensors may contain anywhere from a few hundred to millions of photosites to convert light that is shining on them into small electric currents which can be electronically processed and interpreted and eventually used to recreate and display a target image on a display device.
The processes used to produce these image sensors though highly advanced are not perfect. Photosites in an image sensor array can vary in their sensitivity or ability to convert light into electric currents. Some variation is expected and tolerable, but on occasion, individual photosites in an array can be dead, inactive or over active. This can result in a displayed image having a dark or black dot, bright white dot or incorrect color dot, all of which are unwanted and are undesirable to the end user. Those in the digital imaging field have also recognized the problems caused by improper digitization of target images and have proposed various solutions to detect and correct defective pixels. These prior art methods and systems can be broadly classified into two groups.
On one hand, there are various hardware-based methods for detecting and correcting defective pixels taken from an image sensor array. These hardware-based methods and systems are quite common to video camera manufacturers. Most conventional video cameras using a solid state image pickup device, incorporate a defective pixel detection and correction circuit for correcting defective pixels taken from an image sensor array. The defective pixels are produced when or after the image sensor array is manufactured. In such a camera, a defective pixel correction circuit detects a defective pixel and stores the position data and various data related to the defective pixel in a read only memory (ROM) or the like. Then, when the video camera is in use, pixel data from the defective pixel is replaced by data from a pixel near the defective pixel. One such method and system is disclosed in U.S. Pat. No. 5,796,430. The disadvantage of such approaches is the need to incorporate and program memory devices during product assembly and testing which add costs and delays. Additionally, the hardware device costs will also increase due to the need to add the correcting circuitry or logic to the video processing application specific integrated circuit (ASIC).
On the other hand, there are various software-based methods for detecting and correcting defective pixels in digital images. One such method and system is disclosed in U.S. Pat. No. 5,982,946. Such software-based methods are generally aimed at correcting bad image pixels in an already digitized and fully processed image. These already digitized images are in their final displayable and stored forms which have already been through color processing, compression and various other processing that are performed on the raw data which is read off an image sensor array. Therefore, such software-based methods which treat completely processed digital images, may correct anomalous pixels that are caused by any number of artifacts such as dust or dirt on the original scene, dust or dirt on the lens which was used to capture the scene as well as anomalous pixels in the digitized image which were caused by defective photosites. These methods typically rely on rather sophisticated and expensive systems and computer software to detect and correct bad pixels. These methods generally require user input to detect the location of the potentially bad image pixels. Once the user has visually scanned an entire image and flagged potentially bad image pixels, the system incorporating the software program takes over to correct the flagged defective pixels. Besides requiring user input, such approaches are also expensive, tedious and very time consuming.
The proliferation of inexpensive PC-interfacing digital still and video camera devices requires rapid, dynamic, inexpensive and intelligent defective pixel detection and correction solutions.